A coincidence that is far to outlandish to be real this being coincimental.
"coinci" part of the word coming from coincidence meaning:a striking occurrence of two or more events at one time apparently by mere chance.
And the "mental" part of the word coming from the slang and informal usage: slightly daft; out of one's mind; crazy.
Basically a beefing up of the word coincidence.

You and a friend seperatly go shopping for an outfit for a party. Upon arriving at the party both your outfits are identical. "Thats fucking coincimental mate".